1. Field of the Invention
This invention relates to optical receivers in which an array of discrete photodetectors is employed to sense an input optical signal.
2. Description of the Related Art
Microwave and millimeter wave photodetector receivers are limited in their optical collection areas by the required bandwidth of the system. This limitation stems from the RC time constants of the photodiodes that are typically used for optical reception, since the photodiode depletion region capacitance is directly proportional to the area of its p-n junction. Accordingly, the device area must be scaled down as the desired operational frequency increases. This, however, limits the amount of optical power that can be converted to electrical current by the receiver before the nonlinearity of the conversion process causes unacceptable signal distortion, or the detector itself burns out. Photodetector structures are well known and are described, for example, in Wang, Introduction to Semiconductor Technology: GaAs and Related Compounds, John Wiley & Sons, 1990, pages 482-486.
A photodetection system that purports to achieve higher power capabilities is described in Taylor et al., "Traveling Wave Photodetectors", Optoelectronics Signal Processing for Phased-Array Antennas II, SPIE Vol. 1217, 1990, pages 59-63. In this system an RF electrical transmission line is used as the electrode of an elongated photodetector. To obtain a velocity match between the RF electrical signal and the optical signal, it is suggested that a dielectric overlay be provided on top of the transmission line, or that the transmission line be buried in the GaAs substrate. The latter approach is said to be capable of achieving an almost perfect phase matching between the phase velocity of the electrical microwave signal v.sub.m and the optical group velocity v.sub.o. However, in this reception system the electrical transmission line is an integral part of the optical detector, which prevents the transmission line from being optimized independent of the photodetector.
In De La Chapelle U.S. Pat. No. 5,001,336, issued Mar. 19, 1991, a number of different optical signals are summed electronically by a plurality of photodetectors connected in parallel. Although it is disclosed only in connection with the detection of a number of different optical signals, it might also be possible to use the system to sum portions of a single optical signal for increased optical power handling. However, the described receiver requires a separate optical fiber for each photodetector. Packaging the receiver would therefore be cumbersome, and the multitude of fiber optic pigtails that would be required would reduce the receiver's reliability. Furthermore, the length of each optical fiber would need to be precisely determined to preserve phase coherence between the RF signals from each detector.
A system that is used to modulate an optical signal with a modulating electrical input is described in Walker, "High-Speed III-V Semiconductor Intensity Modulators", IEEE Journal of Quantum Electronics, Vol. 27, No. 3, March 1991, pages 654-667. The system employs a loaded-line traveling-wave modulator in which an RF strip-line electrical transmission circuit is loaded with discrete printed circuit capacitors. The propagation of the electrical modulating signal is slowed so that it matches the propagation speed of the optical signal that is being modulated. The use of the velocity-matched structure is said to result in very high bandwidth-voltage ratios. However, the described system is for modulating, not receiving, optical signals.